1. Field of the Invention
The present invention relates to a technology for removing a foreign substance, such as dust, adhering to a surface of an optical member provided on an optical axis of an optical apparatus, such as an imaging apparatus.
2. Description of the Related Art
In imaging apparatuses, such as digital cameras, which perform imaging of a subject by converting a subject image into an electrical signal, an imaging light-beam is received by an image sensor. Then, a photoelectric conversion signal output from the image sensor is converted into image data. The image data is recorded on a recording medium, such as a memory card. A charge-coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS) sensor can be used as the image sensor.
In such imaging apparatuses, an optical low-pass filter or an infrared cut filter is disposed on a subject side of the image sensor. When foreign substances, such as dust, adhere to the surfaces of the filter, the foreign substances produce black spots in a captured image. Consequently, visual quality of the image deteriorates.
In particular, in digital single-lens reflex cameras with interchangeable lenses, mechanical operating units, such as a shutter and a quick-return mirror, are disposed in the vicinity of an image sensor. Sometimes, foreign substances, such as dust, are generated by the operating units and adhere to the surface of the image sensor or the filter. In addition, when the lens is changed, a foreign substance, such as dust, may enter the camera body from an opening of a lens mount and may adhere to the surface of the image sensor or the filter.
In order to avoid such phenomena, U.S. Pat. Nos. 7,324,149 and 7,006,138 discuss technologies for providing a dust-proof screen, which transmits an imaging light-beam, on a subject side of an image sensor, and for vibrating the dust-proof screen using a piezoelectric element to remove a foreign substance, such as dust, adhering to the surface of the dust-proof screen.
More specifically, U.S. Pat. No. 7,324,149 discusses a camera in which a circular or polygonal dust-proof screen (dust-proof optical member) is disposed between an image sensor and a mechanical shutter and below a finder optical system. In addition, a vibrating member for vibrating a peripheral edge portion of the optical member is provided in the camera and causes the optical member to perform resonant vibrations while switching between first-order and second-order vibration modes each having point-symmetric nodes. Consequently, dust adhering to the surface of the optical member can be removed.
In a first camera discussed in U.S. Pat. No. 7,006,138, a rectangular optical member is disposed between the image sensor and the mechanical shutter and below the finder optical system. Paired piezoelectric elements are respectively attached along longer opposed sides of the optical member. The piezoelectric elements simultaneously vibrate and cause the optical member to perform first-order bending vibrations such that nodes are formed along the vicinity of each of shorter opposed sides of the optical member. Consequently, dust adhering to the surface of the optical member can be removed.
In a second camera discussed in U.S. Pat. No. 7,006,138, a rectangular optical member is disposed between the image sensor and the mechanical shutter and below the finder optical system. Two pairs of piezoelectric elements are respectively attached along longer opposed sides of the optical member. The two pairs of piezoelectric elements are disposed such that a polarization arrangement of an outer-peripheral-side one of the piezoelectric elements of each pair is shifted from the other (inner-peripheral-side one) of the piezoelectric elements. Then, periodic voltages are applied to the piezoelectric elements of each pair, respectively, by phase-shifting the voltages. Thus, a progressive wave progressing in a longitudinal direction of the optical member is generated. Consequently, dust adhering to the surface of the optical member can be moved.
However, the camera discussed in U.S. Pat. No. 7,324,149 requires that the image sensor, which generally has a rectangular imaging area, is provided with a dedicated dust-proof screen shaped in a circle, whose diameter is longer than a diagonal-line of the rectangular imaging area, or a regular polygon whose size is equivalent to that of the circle. Thus, a space to be wasted in design of the finder optical system is needed. In addition, there is a fear of reduction in optical performance of the camera.
The first camera discussed in U.S. Pat. No. 7,006,138 has a relatively high cost, because the two piezoelectric elements are attached along a pair of parallel sides of the rectangular optical member. In addition, because the first camera uses the first-order vibration mode, a difference in amplitude of the vibration between a central portion and a peripheral portion of the optical member is large. Thus, a difference in effects of removing a foreign substance between such portions of the surface of the optical member is likely to occur. The effects of removing a foreign substance vary with an outer shape, a thickness, Young's modulus, a density of the optical member. In a case where the first-order vibration is used, a resonant frequency may generate a low operation sound.
The second camera discussed in U.S. Pat. No. 7,006,138 also has a relatively high cost, because the second camera requires the two pairs of piezoelectric elements, i.e., four piezoelectric elements. In addition, because a progressive wave is generated by applying the phase-shifted periodic voltages to phase-shifted electrodes of the piezoelectric elements, a control process itself and circuits for performing the control process are complicated. In addition, an adjustment process for absorbing variation at production of cameras may be required.